Control of the combustion process in a combustion chamber of a gas turbine is essential for the performance of a gas turbine and the operating lifetime of its components. A control is furthermore crucial to assure that emissions in the turbine exhaust gases of environmentally harmful gases, as for example NOx, remain within the limitations determined by authorities.
Numerous methods are known for controlling the combustion process and the flame characteristics, as for example the flame temperature and spatial profile, by specific control of the fuel mass flow, temperature, or type, controlled injection of air, or the introduction of water droplets into the combustion chamber.
In order to control the combustion and the flame in the combustion chamber, it is of particular importance to have knowledge of the temperature and pressure within the chamber as well as of the concentrations of water, carbon dioxide, and other gases.
One known method is the use of a thermocouple to determine the temperature and to feed the temperature value into the control loop of the combustion chamber. While this can provide the necessary temperature information, the method is not very suitable for use in gas turbines in the field because the measurement data develop a temperature drift requiring frequent and tedious re-calibrations. Furthermore, the thermocouples, when applied in the high temperature ranges of 1350° C. and above, have a limited operating lifetime and therefore have to be frequently replaced.
Instead of thermocouples, optical sensing methods can be used for the measurement of temperature. These methods mainly include laser absorption spectroscopy, which uses a tunable diode laser source (TDLAS) and transmitting and beam shaping optics that direct the laser light of a certain frequency spectrum through an optically accessible absorbing medium. The tunable laser source is for example a VCSEL or DFB laser, which is tuned over the spectrum encompassing the absorption lines of the components of the medium to be analyzed. Receiving optics and a detector such as a photodiode and a data analysis unit allow determination of not only the temperature, but also concentrations and pressures of a transparent and absorbing medium such as a gas or liquid. Laser absorption spectroscopy is however limited in its applications regarding the pressure of the medium to be measured because at high pressures above 5 bar the absorption lines are broadened, also known as Voigt broadening, which leads to a convolving of the absorption lines and consequent loss of resolution.
WO2009/052157 discloses an apparatus, for performing laser absorption spectroscopy on a gaseous medium, having transmitting and receiving optics, of which at least one is translationally movable. Absorption spectra can thereby be obtained sequentially over a variety of optical paths within the time of movement of the optics. A spectral analysis of the data allows determination of gas type, temperature, and quantity.
EP2107305 discloses sensors sensor placed downstream of the flame in a gas turbine combustion chamber for the measurement of the oxygen concentration. The flame temperature is then determined based on the oxygen concentration value. The oxygen sensor is placed in particular at a location of maximum temperature.
US2008/0289342 describes the difficulty of placing measurement devices in a combustion chamber of a gas turbine due to the harsh environmental conditions, particularly in regard to temperature and pressure. The laser beam of a tunable diode laser is coupled to a fiber and transmitted through the combustion chamber via a port in the outer casing. Optical signals of at least one IR-wavelength are detected and analyzed using a computer to determine the concentration of at least one combustion species. A computer is used to generate engine control input parameters as a function of the concentration of the at least one combustion species. Optical signals of different wavelengths are measured in order to determine the concentration of H2O, CO, or CO2.